1. Field of the Invention
The present invention relates generally to a wideband code division multiple access (W-CDMA) mobile communication system, and in particular, to an apparatus and method for testing a voltage standing wave ratio (VSWR). More specifically, the present invention relates to an apparatus and method for testing the VSWR without affecting a call quality of the W-CDMA mobile communication system.
2. Description of the Related Art
In general, a code division multiple access (CDMA) mobile communication system is classified into a wideband CDMA (W-CDMA) mobile communication system and a narrowband CDMA (N-CDMA) mobile communication system based on the bandwidth used for a communication service. The W-CDMA mobile communication system and the N-CDMA mobile communication system both provide a communication service using high-frequency radio signals. Since the W-CDMA mobile communication system and the N-CDMA mobile communication system both use high-frequency radio signals to provide a communication service, the uniformity of a high-frequency transmission line is an important measure for determining the quality of the communication service. Typically, a voltage standing wave ratio (VSWR) is used as a criterion for determining the uniformity of the high-frequency transmission line. The VSWR is defined as a ratio of transmission signals transmitted in the form of voltage and current to a wave generated by reflected transmission signals when the high-frequency transmission line is non-uniform. That is, the VSWR is a type of return loss. As the VSWR becomes lower, the high-frequency transmission line becomes more uniform. For example, a VSWR of 1.2 represents a return loss of about −20 dB. In addition, a device for measuring the VSWR is generally called a “radio frequency (RF) monitor”. The RF monitor selects one of an actual signal provided from a base station (BS) and a generated test signal in order to measure the VSWR. Herein, a description will be made of an apparatus and method for measuring a VSWR using the test signal in the case where an antenna front end unit (AFEU) for reception is used.
A description will now be made of a RF monitor, i.e., a VSWR measurement apparatus, with reference to FIG. 1.
FIG. 1 is a diagram illustrating an example of a general VSWR test apparatus. Referring to FIG. 1, a VSWR test apparatus 100 includes a test signal generator 111 for generating a test signal for a VSWR test, a VSWR detector 113 for detecting a VSWR according to the test signal, and a controller (not shown). Upon detecting a VSWR test request for testing the VSWR, the controller provides a test signal generation request signal to the test signal generator 111. In this case, the controller determines the oscillation frequency information and power level information, representing an oscillation frequency information and power level information, respectively, at which the test signal should be transmitted. Further, the controller generates the test signal generation request signal including the determined oscillation frequency information and power level information. In addition, the controller considers the following factors in determining the oscillation frequency information and power level information used for generation of the test signal.
The VSWR test apparatus 100 of FIG. 1 is an apparatus for testing VSWR in an N-CDMA mobile communication system. However, the N-CDMA mobile communication system employs a plurality of frequency assignments (FAs) for a communication service. Therefore, the controller detects a currently unemployed FA among the plurality of FAs, and controls the test signal generator 111 so that the test signal oscillates at a center frequency of the detected unemployed FA. As a result, the oscillation frequency information of the test signal generation request signal generated by the controller becomes the center frequency information of the currently unemployed FA. Further, in determining the power level information, the controller determines the power level information so that the generated test signal is higher by a prescribed level than the power level of signals received at the N-CDMA mobile communication system.
Upon receiving the test signal generation request signal, the test signal generator 111 detects the oscillation frequency information and the power level information included in the test signal generation request signal, and generates a test signal in response to the detected oscillation frequency information and power level information. The generated test signal includes a continuous wave (CW) format of single tones which correspond to the oscillation frequency information. Since the test signal generated by the test signal generator 111 must be transmitted via a first path to an antenna 160 and via a second path to the VSWR detector 113, the test signal generator 111 outputs the test signal through a first port which corresponds to the first path and a second port which corresponds to the second path. The test signal generated by the test signal generator 111 includes a single tone format, illustratively, represented by (3) of FIG. 1.
The test signal outputted through the first port is provided to a first port of an AFEU 150. The test signal outputted through the second port is provided to a second port of the AFEU 150. The AFEU 150 provides the test signal received through the first port to the antenna 160, and provides the test signal received through the second port to the VSWR detector 113 via a third port. The test signal provided to the antenna 160 is not fully transmitted. A portion of the signal is reflected due to a characteristic of a radio link. The reflected signal is provided to the VSWR detector 113 via the third port by the AFEU 150. The signal reflected from the antenna 160 is not fully provided to the VSWR detector 113 via the third port. Rather, a portion of the signal is provided to a reception terminal via a fourth port. A signal received through the antenna 160 has a continuous wave format of FAs, illustratively, represented by (1) of FIG. 1. Ideally, only the FAs received over the air through the antenna 160 must be provided to the reception terminal. However, as stated above, a part of the test signal for the VSWR test is provided to the reception terminal. Therefore, the FAs overlap with the test signal as, illustratively, represented by (2) of FIG. 1.
The VSWR detector 113 calculates a VSWR, using signals outputted at the third port of the AFEU 150, i.e., the test signal directly provided from the test signal generator 111 to the VSWR detector 113 and the test signal reflecting from the antenna 160 after being provided from the test signal generator 111 to the antenna 160.
Since the reflected test signal is not ideally fully provided to the VSWR detector 113 due to being partially provided to the reception terminal, a power level of the test signal must be determined for a power level of a received signal as stated above.
Now, with reference to FIG. 2, a description will be made of a form of signals used for a VSWR test described in conjunction with FIG. 1.
FIG. 2 is a graph illustrating an example of a VSWR test signal of FIG. 1. Referring to FIG. 2, a VSWR is tested on the assumption that an N-CDMA mobile communication system employs N FAs. Further, it will be assumed that among the N FAs, i.e., first to Nth FAs F1, to FN, a fourth FA F4 is currently unemployed. The test signal generator 111 then transmits a test signal T1, for measuring the VSWR at a center frequency of the FA F4. When the FAs include no currently unemployed FA, the test signal generator 111 transmits the test signal T1, at a center frequency of a particular one of the FAs in use. Since the N-CDMA mobile communication system is generally used for voice communication, it is possible to transmit the test signal T1, at the center frequency of the particular FA in employment. That is, in the case of voice communication, even though a voice data loss occurs, service quality is not fatally affected by the voice data loss. Further, if it is assumed as illustrated in FIG. 2 that received FAs have a power level of P1 dB, the test signal T1 has a power level of P2 dB which is higher by a prescribed level than the P1 dB, considering a noise component.
As described above, the N-CDMA mobile communication system oscillates a VSWR test signal at a center frequency of a current unemployed FA among its employed FAs, for a VSWR test. Alternatively, if there exists no currently unemployed FA, the N-CDMA mobile communication system oscillates the VSWR test signal at a center frequency of a particular FA currently in employment, for a VSWR test. However, when the test signal is oscillated at a center frequency of a currently employed FA as described in conjunction with the N-CDMA mobile communication system, a fatal error may occur in packet data that is nonvoice data. That is, the packet data suffers a loss due to transmission of the test signal. The loss of packet data disables normal data demodulation. However, since a W-CDMA mobile communication system generally provides a data service as well as a voice service, the system may be fatally affected when a test signal is oscillated at a center frequency of a currently employed FA as stated above. In addition, unlike the N-CDMA mobile communication system, the W-CDMA mobile communication system employs fewer FAs, for example, 4 FAs. Therefore, most of the FAs are actually employed for a communication service, so it is difficult to assign a currently unemployed FA for transmission of the test signal.
In addition, in the W-CDMA mobile communication system, a test signal for measuring the VSWR must be transmitted at a power level that satisfies the following three factors without affecting the actual quality of the communication service.
First, in the W-CDMA mobile communication system, consideration should be taken of a noise floor level of a signal received at an antenna of a Node B. For example, assuming that a noise floor level is −108 dBm/3.84 MHz in the W-CDMA mobile communication system, the test signal must be transmitted at a power level of over −129 dBm/30 kHz+20 dB (return loss)=−109 dBm/30 kHz in order to transmit the test signal at a power level higher than the noise floor level.
Second, in the W-CDMA mobile communication system, consideration should be taken of a peak power level of a signal received at a Node B receiver. For example, assuming that a peak power level of a signal received at a Node B receiver is −60 dBm/3.84 MHz in the W-CDMA mobile communication system, the test signal must be transmitted at a power level which is higher by at least −61 dBm/30 KHz than the peak power level of the received signal.
Third, in the W-CDMA mobile communication system, consideration should be taken of a data rate provided by a Node B. For example, in the case where a data rate provided by a Node B is 384 Kbps in the W-CDMA mobile communication system, sensitivity of a received signal is −106 dBm/3.84 MHz. In this case, if a resolution bandwidth (RBW) is set to 30 KHz, a power level of the test signal must be set to below −127 dBm/30 KHz in order not to interfere with transmission of other data.
However, a VSWR test apparatus used in an existing N-CDMA mobile communication system tests a VSWR without considering the above three factors, i.e., the power level consideration system, so the VSWR measurement of the W-CDMA mobile communication system, so the VSWR test apparatus is not proper for VSWR measurement of the W-CDMA mobile communication system. In addition, as described above, the VSWR test apparatus used in the existing W-CDMA mobile communication system generates a test signal at a frequency band of a currently unemployed FA among the employed FAs. However, in the case of the W-CDMA mobile communication system, the number of actual FAs is limited, so it is impossible to assign a currently unemployed FA for the VSWR test. Accordingly, there is a need for a new method of measuring a VSWR in the W-CDMA mobile communication system.